The physical-chemical properties and heterogeneities of lipid membranes, which are important to many biological processes, are being investigated by atomic force microscopy (AFM) in this project with NIAID collaborators. Quantitative features of the main phase transition in 2-dimyristoyl-sn-glycero-phosphocholine (DMPC) have been resolved by tapping-mode AFM images in an environmentally controlled chamber at various temperatures. These AFM images reveal two membrane phases near the expected DMPC chain-melting temperature. We have quantified the marked thinning and mechanical softening of the DMPC membrane upon chain melting from precise AFM thickness measurements. We have also constructed a novel biophysical model, which permits an estimate of the thermodynamic transition enthalpy, entropy, and the membrane phase domain size from our AFM-acquired temperature-dependent phase distributions. The deduced intrinsic domain size is about 4.2 nm in diameter. The work on DMPC membrane has been extended to examine the more biological membrane microdomains, or rafts, in (1) tri-lipid mixtures of dipalmitoyl phosphatidylcholine (DPPC), dilauroyl phosphatidylcholine (DLPC), and cholesterol (Chol), and (2) red blood cell membranes. We found that the tri-lipid membrane formed both microscopic and nanoscopic domains over mica substrates; both the cholesterol level and the temperature affected the sizes and dynamic of these domains. We developed new video microscopy and image analyses to characterize the red blood cell flicker and edge dithering phenomena during the Plasmodium falciparum malaria infection process. We found that the parasitic infection markedly modifies cell membrane dynamics, in potential relevance to malaria disease mechanism in microcirculations. Overall, the AFM and related technology are being advanced further to elucidate membrane-associated biomolecular events critical to medically important processes.